The invention relates to a process for the production of ammonia NH3 under pressure from synthesis gas according to the reaction (1):3 H2+N2→2 NH3  (1)which contains the reactants hydrogen H2 and nitrogen N2 as well as ingredients which are usually inert to reaction (1) such as methane and noble gases which impede the conversion rate of the reaction (1) and which will hereafter be referred to as “inert ingredients”. Processes of this type are usually operated in such a way that the make-up gas is first compressed in several stages to a high pressure and the compressed make-up gas then fed to a loop which encompasses one or more reactors filled with catalysts to produce ammonia. The loop is provided with an outlet to withdraw liquid ammonia which has been produced.
In order to avoid an enrichment in the loop of the inert ingredients which are contained in the withdrawn ammonia and which are only soluble at very low concentrations, a partstream of the gases circulated in the loop is continuously withdrawn as purge gas. The residual ammonia is removed from this purge gas by scrubbing, the hydrogen and the nitrogen, if any, being removed and recovered by using membrane technology or low-temperature separation. The residual inert ingredients such as methane, argon, helium and residual nitrogen, if any, are discharged or otherwise used for heating purposes. The recycle gas is added to the make-up gas before it is compressed and thus re-used. It is detrimental to the energy balance to withdraw large amounts of purge gas from the loop since this would cause a significant drop in pressure for large volumes of gas which must then undergo secondary compression with much expenditure incurred.
This is the reason why the enrichment of the inert ingredients from an original value of 1 to 2% by volume in the make-up gas up to 10 or 20% by volume cannot be avoided within the recycle gas, even though there is the inevitable disadvantage associated with these high inerts concentrations that the partial pressure of the gases participating in the reaction, which alone are crucial for the state of the reaction equilibrium as affinity to the reaction, are significantly lower than they would be in a completely inerts-free synthesis gas loop. This is the reason why the volume of the catalysts used and the reactors which house them must be significantly larger than would be required without the presence of inert ingredients in the synthesis gas loop.
The enrichment of inert ingredients in the loop compared to the original level of concentration in the make-up gas, which is tolerated despite the disadvantages described above, demonstrates the technical paradox which arises in that the operating costs, particularly the compression costs, decrease in the presence of smaller amounts of purge gas and thus of higher concentrations of inert ingredients, while the capital costs increase due to the larger catalyst volumes required, or the need of using alternatively more expensive catalysts such as those based on ruthenium. This technical paradox cannot be resolved using current state-of-the-art technologies. The specialist in this field is thus compelled to find a compromise and to establish the optimum cost balance in respect of high operational expenditure and capital cost.
The synthesis taking place in the reactor yields product gas from the synthesis gas. It primarily consists of the unreacted portion of the feed gas, the ammonia formed and the inert ingredients. The ammonia is gaseous at the reactor outlet but it must be condensed so that it can be separated from the product gas and be withdrawn as liquid ammonia from the loop. Since the dew point of ammonia depends on its partial pressure and its temperature, it is an advantage to the condensation of the product to provide a higher synthesis pressure and a high ammonia concentration on the one hand, while having a lower temperature on the other. A high ammonia concentration can be obtained by using large catalyst volumes at low inerts concentrations. A high synthesis pressure leads to a correspondingly higher cost of energy required to compress the synthesis gas and a lower cooling temperature demands that an appropriate cooling apparatus be installed in the recycle gas.
A working synthesis pressure of between 150 and 280 bar is usually selected to provide cooling for the ammonia produced in the loop by means of heat transfer systems, water or air cooling and/or additional cooling circuits, and they are combined in such a way that the ammonia condenses and can thus be withdrawn in the liquid phase. This relatively high pressure has the advantage that a large portion of the ammonia already condenses at relatively high temperatures such as those that can be achieved by water cooling (e.g. cooling down to 30–40° C.). For example, the dew point of a product concentration of 20 molar % at a 200 bar synthesis pressure equals about 57° C. A water cooling system rated for 35° C., for example, allows the ammonia content in the gas to be lowered to 11.2 molar % and thus permits a recovery of 59% of the condensable product portion. The fact that the recycle gas returned to the reactor should have as low an ammonia concentration as possible (in our example 3.8 molar %) necessitates, as a rule, the downstream installation of an additional refrigeration circuit using water in order to make an even larger part of the product condense at even lower temperatures (e.g. cooling down to a temperature of between −10° C. and 0° C.). This refrigeration circuit requires energy to recompress the vaporised refrigerant. The portion of the product which requires condensation through refrigeration is small when applying the above-mentioned synthesis pressures, which means that a correspondingly small amount of energy is required for the refrigerant compressor.
These above facts reveal the reasons why the specialist in the field normally tends to maintain the working synthesis pressure between 150 and 280 bar but exceptions are known in which the synthesis pressure was reduced to 60 bar. Patents EP 000 993, EP 093 502 and EP 179 392 describe such configurations. Since the volume of conventional magnetite catalysts would grow disproportionately if the synthesis pressure were lowered and since this also applies to the constructional requirements for the reactors, the processes described use highly active catalysts. Magnetite catalysts doped with cobalt are still required in large amounts, the ruthenium catalyst being more expensive because of its noble metal content.
The lower the synthesis pressure the lower the amount of heat which can be dissipated by using water or air cooling and consequently the portion of heat to be removed by refrigeration increases accordingly. This leads to a further technical paradox if one considers, as in standard practice, that the refrigeration requires a cooling circuit with a compressor set. While the compression expenditure for the synthesis loop declines as the synthesis pressure decreases, the compression expenditure for the cooling circuit increases since more refrigeration is required to withdraw the ammonia produced in the synthesis loop. The portion of ammonia condensed prior to refrigeration is increased in low-pressure processes in that a very low concentration of inert ingredients is set by means of a high flow rate of the purge gas stream. The problem with the enrichment of inert ingredients occurs as in the high-pressure synthesis process and a lower inerts concentration increases the product concentration and consequently the dew point. Hence, the specialist in the field must in this case, too, find a compromise and establish an optimum cost balance in respect of high operational expenditure and investment costs.
For many years commercial scale production of ammonia has been undertaken in large single-train plants. The single-train concept is the result of the high costs associated with a loop operated at high pressure and of the high costs for the compression process, which both are subject to high degression with increasing flow rates. Hence, some sort of technical prejudice has been held for many years according to which the economical production of ammonia is feasible only in single-train chemical plants.
Only few attempts have been made to add another train to such a single-train plant as, for instance, described in DD 225 029. This document describes two high-pressure synthesis units arranged one after the other and operated at the same pressure levels, the first being a make-up gas system and the second a conventional loop system, the aim of said concept being to reduce the amount of ammonia produced in the loop by the amount produced in the upstream make-up gas system in order to reduce the loop volumes accordingly. The system has not been able to find acceptance on the market over the last 20 years, a fact which appeared to confirm the existing prejudice that only single-train plants can operate economically. Moreover, the said technical paradoxes also apply in full to the reaction system described in DD 225 029.